1. Field of the Invention
The invention relates to a process for spinning a polymer filament. More particularly, the invention relates to a process for forming a polymer filament in an electrostatic charged field provided by plural, spaced electrodes. The invention also relates to a polymer forming apparatus comprising a spinneret.
2. Discussion of the Related Art
The invention relates to a process for the production of polymer filaments, fibers and other very fine polymer extrudates from a thread-forming polymer. A solid polymer is converted to a fluid state so that it will pass under pressure through a fine extrusion die orifice. A continuous liquid phase filament is extruded and drawn though a zone in which solvent evaporation and cooling takes place, causing the filament to solidify and form a continuous, solid filament. The solid filament is collected by means such as a rotating drum, moving belt, water bath, and the like or a combination thereof. In a filament extrusion apparatus, the die orifice is oriented to pass the liquid phase filament directly toward the collection means. This facilitates a linear orientation in the solidifying polymer filament. Drawing and annealing also facilitate the linear orientation of the fiber.
In the electro spinning process, solvent evaporation from the filament, filament cooling or both take place in the zone between the extrusion die and the filament collection means. This zone is biased to maintain an electrostatic field.
The extrusion die and the filament collection means are each electrically conductive. An electric potential difference is maintained between them. In commercial practice the bias is in the range of 5,000 to 15,000 volts, often 5,000 to 10,000 volts. An electro spinning apparatus usually has a positively biased die and a grounded collection means. A positively biased die with a negatively biased collection means has also been used. The polarity necessitates a minimum separation between the die and collection means to prevent arcing across the zone. In this sense, the electrostatic field is elongated or longitudinal between the two conductors. Reference to the elongated or longitudinal electrostatic field is also consistent with the longitudinal orientation of the filament as it is drawn linearly in the electrostatic field.
The polymer filament is drawn from the die orifice to the collection means under the influence of the electrostatic field. As a result, the extruded molten filament is subjected along its length to an electrostatic field. The strength of the electrostatic field decreases exponentially with the distance between the electrodes. Accordingly, the drawing rate from the anodic die to the cathodic drawing means varies along the filament length due to the variation in the electrostatic charge on the filament.
U.S. Pat. Nos. 1,975,504; 2,158,416; and 2,323,025 to A. Formhals disclose a process and apparatus for preparing artificial threads. These pioneer patents disclose the essential elements of the process, including a pair of spaced electrodes. These electrodes produce an electrostatic field through which an extruded filament is drawn. Means is provided for varying the rate of filament collection. The filament is spun into yarn by mechanical spinning means.
In accordance with the invention, a process is provided for electro spinning a polymer filament, fiber or the like in an electrostatic field created between a biased polymer extrusion die and a collection means.
A thread-forming polymer is liquefied and extruded through a die orifice. The extruded filament is drawn while solidifying, through the electrostatic field. A solid filament is collected on the collection means.
The electrostatic field is sequentially biased along the length of the liquid filament. The polarity of the electrostatic field is the same as that of the die. The extruded filament is exposed to the sequential bias as it solidifies. As a result, the liquid state filament and the resulting solid filament product have a uniform linear molecular orientation longitudinally along the filament. Therefore the solid filament product tends to have a uniform linear molecular orientation.
Solid filaments produced according to the process has physical properties which make them particularly useful for making protective clothing such as chemical and biological protective clothing, light weight personal body armor and the like. The solid filament also has utility in applications in uses such as tissue growth medium, particulate filters and for optical and electronic applications.